I know I have seen this somewhere on here, but I can't find it. Right now I have an MC2100 installed and I have a rich mixture I'm trying to solve. I have installed #46 jets and now the carb floods, to the extent that the bowl overflows. This didn't happen with #48 jets, only that the engine ran so rich that I am experiencing some washing of the cylinder walls. Now to the question. My fuel return line builds slight pressure when you blow into it. I doesn't when the gas cap is off. How does the vent line back to the charcoal canister work in conjunction with the fuel return line? Should both lines "breathe" freely? I have recently replaced the power valve. Thanks.

Went through the same problem. Sounds like jeffy is right - there may be an obstruction between the tank, the check valves, and the canister. My valve-canister line was plugged with charcoal when I tore down the system. That link will give you a pretty solid idea of how the whole fuel emissions/canister system is supposed to work.
-N